1. Field of the Invention
The present invention relates to methods and structures for attaching a semiconductor chip or chip carrier to a substrate and, more particularly, to methods and structures for attaching a semiconductor chip or chip carrier to a substrate using solder ball technology.
2. Background and Related Art
In the fabrication of electronic devices as, for example, during ball attach or card attach, low melt C4 (controlled collapsed chip connection) solder balls on a chip carrier will reach their melting temperature and become liquid. Typically, for solder with a high tin content, the volume expansion associated with this phase change can range between 3 and 6%. If the C4 solder balls have been encapsulated prior to this volume change, as is typically the case, the volume expansion is constrained and the resulting pressure may result in the squeezing of this expanding volume of liquid into voids present in the surrounding underfill and its associated interfaces. This volume expansion of solder may also result in opening any weak interfaces, like underfill to chip passivation (for example polyimide) or underfill to solder mask interfaces. It is clear that the effect of such action could result in device failure.
In accordance with the present invention, structures are provided on the chip carrier to relieve pressure created by volume expanding solder upon heating and reflow. The structures are formed directly beneath the solder balls or bumps. The pressure relief structure may be in the form of microchannels or vias, an air cushioned diaphragm, or porous or compressible medium, like foam. The various structures act in a manner to accept or accommodate the expanding or excess volume of solder created during melting to thereby minimize or avoid the creation of pressure that may affect the region adjoining or surrounding the solder balls and the various material interfaces.
Accordingly, it is an object of the present invention to provide improved methods of making connections in electronic devices, to enhance overall reliability of the product.
It is another object of the present invention to provide structures which act to accommodate expanding solder when it changed to the liquid phase.
It is yet another object of the present invention to provide a method of attaching enclosed solder balls to connection pads by providing structures that accommodate expanding solder upon reflow.
It is a further object of the present invention to provide structures that relieve internal pressures in an enclosed electronic packaging environment caused by the expansion of solder when going from the solid to liquid phase.
It is yet a further object of the present invention to provide methods and structures that relieve pressure from solder reflow to thereby prevent damage to material interfaces in electronic devices.